Among the parameters that must be known and controlled in order to enable the fabrication of high quality semiconductor films and devices, and integrated circuits, are included the thicknesses of constituent layers and the concentrations of free carriers present therein. Techniques are known for determining those parameters, and are implemented in commercial instruments; as far as is known, however, no such instrument is entirely satisfactory, for any of several reasons.
Certain instruments presently available measure an interferogram of the material being evaluated, rather than utilizing a direct spectral analysis. A primary deficiency of such a method lies in its inability to provide information concerning the free carrier concentration, or doping density, of the material. A second deficiency resides in its inability to provide precise thickness measurements for thin layers and for layers with small differences in free carrier concentration, as compared to the substrate. Other instruments that are designed to measure epitaxial silicon layers by use of a representative interferogram, and that measure doping density in dielectric films by use of absorption peaks, suffer from similar deficiencies.
Ellipsometers are available which utilize a single frequency, or a narrow frequency range, to obtain thickness measurements. They cannot however separate the variation of thickness from the variation of dielectric function, and assumed dielectric constants are employed to derive thickness values, which may lead to inaccuracies. Such ellipsometers are moreover incapable of obtaining precise thickness information from complicated systems and nonuniform films, such as unannealed polycrystalline films, patterned wafers, and materials exhibiting dielectric functions that are strongly dependent upon frequency.
Certain other instruments require operation under ultra-high vacuum conditions, making product wafer characterization during fabrication impossible to achieve as a practical matter; also, they tend to perform relatively slowly and to be inordinately expensive. Although Secondary Ion Mass Spectrometers "SIMS" are capable of obtaining doping density profiles, they actually determine chemical, rather than active free carrier, concentrations, and they are not capable of distinguishing between annealed and unannealed samples; moreover, such apparatus operates ex-situ, and destructively.
The need is well known for accurate measurement of thickness and doping density information in the production of devices for the microelectronic industry. In an article entitled "Epifilm Thickness Measurement Using Fourier Transform Infrared Spectroscopy; Effect of Refractive Index Dispersion and Refractive Index Measurement" (J. Appl. Phys. 76(4), 15 August 1994), Zhou et al provide procedures and results of refractive index measurements, performed on both lightly and heavily doped silicon samples over the mid-infrared spectrum region. A strong dependence of refractive index, as a function of substrate dopant concentration, is reported, as is a significant variation with wavelength of the refractive index of heavily doped silicon material. The Zhou et al technique utilizes however only the real part of the refractive index, and a value of .epsilon.* (the complex dielectric function) equal to the square of the refractive index. As a result, not all electric inactive modes are taken into account, which in turn leads to a degree of inaccuracy; also, the technique cannot compare measured and calculated spectra directly, and there is no suggestion for determining transition width values.
In a paper entitled "Infrared Reflectance Spectra of Thin-Epitaxial Silicon Layers" (SPIE Vol. 276, Optical Characterization Techniques For Semiconductor Technology (1981), pages 222 through 226), Senitzky et al discuss the investigation of IR reflectance spectra of thin expitaxial layers on substrates containing n-type buried layers. They report finding that the Drude model, with a constant relaxation time, should be used to compute the optical constants of the buried layers. The re-flectance spectra can then be used to determined epitaxial layer thickness, and maximum carrier concentration and diffusion width of a buried layer. The Senitzky et al method is also limited by its failure to take all electric inactive modes into account, as well as by a requirement for using heavily doped buried layers.
Milosevic et al report, in an article entitled "Applications of the Theory of Optical Spectroscopy to Numerical Simulations" (Applied Spectroscopy, Volume 47, Number 5, 1993, pages 566 through 575), development of a numerical simulation to extract the optical constants from experimental spectra. Potential applications for the method are said to include determining film thicknesses; no practical development or application for multilayer structures is provided, however, and free carrier concentration factors are not addressed.
A Bio-Rad Digilab Division report on epitaxial thickness measurements, authored by Krishman et al, discloses the use of FTIR spectrometers for the accurate determination of epitaxial layers, inclusive of silicon-on-silicon, silicon-on-sapphire, gallium arsenide-on-gallium arsenide, and mercury-cadmium-telluride-on-cadmium telluride. Because however all calculations employ interferograms, analyses are limited to thicknesses in excess of 1 .mu.m; also, the technique is incapable of determining free carrier concentrations.
Characterization of thin solid films and surfaces by FTIR spectroscopy, based upon reflectance and transmittance measurements, is described by Grosse in an article entitled "FTIR-Spectroscopy of Layered Structures--Thin Solid Films, Coated Substrates, Profiles, Multilayers" (SPIE Vol. 1575, 8th International Conference on Fourier Transform Spectroscopy (1991), pages 169 through 179). Observed reflectance and transmittance are simulated by spectra, calculated from a model by an optimum fit of the model parameters, thereby characterizing the specimens in terms of vibronic resonances, contribution of free electrons and holes, thicknesses of the various layers in a stack, and profiles of chemical composition. The Gross article is however too general in certain significant aspects, such as in regard to the selection of appropriate parameters within the Drude model, to provide a practical analysis methodology.